Controller for use with an electric screwdriver

ABSTRACT

An object of the present invention is to provide an electric screwdriver device having a function for determining that a correct tightening operation has been carried out when the tightening operation has taken a time longer than a reference time predetermined for the tightening operation, characterized in comprising a novel feature for determining the reference time. The electric screwdriver device of the present invention comprises a torque monitor ( 40 , S 4 ) for monitoring whether or not a load torque in carrying out the tightening by using the electric screwdriver  12  has reached the tightening completion torque, a timer ( 40 , S 1 , S 5 ) for measuring the time consumed from the starting of the tightening operation until the tightening torque has reached the tightening completion torque, as the tightening operation time, and a feature ( 40 , S 6 -S 10 ) for determining the reference time for a subsequent tightening operation based on a plurality of tightening operation time obtained from the timer in the tightening operations carried out for the predetermined number of screws.

This application is a divisional application of U.S. patent applicationSer. No. 11/712,111 filed Feb. 28, 2007, now U.S. Pat. No. 7,464,769,which is a continuation of PCT/JP2005/014311, filed Aug. 4, 2005, whichclaims priority to Japanese Application No. JP2004-250757, filed Aug.30, 2004. The entire contents of these applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric screwdriver and acontroller thereof.

2. Description of the Related Art

There has been provided one type of electric screwdriver equipped with acontroller having a function for preventing any screws from being leftnot tightened. The electric screwdriver, in this type, is connected tothe controller, so that when a tightening torque has reached a valueequal to or greater than a predetermined value in a tighteningoperation, a signal indicating the completion of screw-tightening issent from the electric screwdriver to the controller, and then thecontroller causes a specific counter (a counter for counting a number ofscrews tightened) to increment by one to provide a count indication ofthe number of tightened screws or inform a situation that the number oftightened screws has reached a predetermined one (see, for example,Japanese Patent Application Publication No. 2000-47705, Japanese PatentApplication Publication No. Hei7-80128 and Japanese Patent ApplicationPublication No. 2003-123050.

In the tightening operation using the electric screwdriver, however,there might be an event that a screw has been tightened out of upright.If a screw is tightened out of upright, typically the tightening torquecould reach the predetermined value in a shorter time than in atightening operation normally carried out. Also, when the screw that hasbeen once tightened is tightened again (for additional tightening), thetightening torque could reach the predetermined value in a shorter timethan in a tightening operation normally carried out. Due to this fact, atypical (in most cases, minimal) tightening time expected in the normaltightening operation is previously set as a reference time in thecontroller, such that if the signal indicating the completion of thetightening is generated by the electric screwdriver in a shorter timethan the reference time, then the controller can determine that thescrew must have been tightened in the inclined state or a once tightenedscrew has been tightened again (for additional tightening).

To determine the aforementioned reference time, it is conventional thata test of tightening is carried out on a certain number of screws priorto a practical tightening operation, in which a time consumed forproperly operating screw-tightening is measured by using, for example, astopwatch, and specifically a shortest time in the time measurements istaken as the reference time. Such a tightening test is, however, ahighly cumbersome work for a user.

There is another possible event in the practical operation, a screwmight be tightened at an incorrect location, and in that case, anoperation (manipulation) for inversely turning the electric screwdriveris carried out to untighten the screw. In such a case, in response tothe operation for inversely turning the electric screwdriver, a counterfor counting a number of tightened screws is needed to be reset so as tocancel the count in the incorrect tightening operation. Regarding thispoint, a device according to the prior art has following problems.

FIG. 9 shows a typical wiring diagram in an electric screwdriver Dcomprising a controller P with a function for preventing any screws frombeing left not tightened. In the drawing, “D1” and “D2” designate powersource lines to a screwdriver motor M, “D3” is an start/stop signalline, “D4” is a tightening completion signal line and “D5” is a commonline. The common line is connected to a reference voltage Vs. The powersource lines D1 and D2 are connected to a power supply circuit S of thecontroller P. The screwdriver motor M is a type of direct-current motorand the power circuit S comprises a direct-current power source. “RS”designates a normal/reverse rotation select switch for changing thepolarity of the power source output voltage in order to select therotation direction of the motor M (normal or reverse rotation). “SS”designates a starting switch, which in response to an open/closeoperation by an operator, sends a start/stop signal to the power circuitS via the start/stop signal line D3. For example, when the startingswitch SS is closed, the reference voltage Vs from the common line D5 isapplied to the power circuit S as the start signal, whereas when thestarting switch is opened, the reference voltage Vs from the common lineD5 is shut off, and the stop signal is sent to the power supply circuitby the shut-off. “BS” designates a braking switch of a normally opentype, which is shifted to a closed position by an action of a cam Cincorporated in the screwdriver when the load torque in the tighteningoperation has reached a predetermined value. When the braking switch BSis closed, a tightening completion signal at the reference voltage Vs issent to a counter T for counting the number of tightened screws and alsoto the power supply circuit S in the controller P via the tighteningcompletion signal line D4.

The circuit shown in FIG. 9 is not able to provide the signal from theelectric driver D to the controller P to reverse the count in thecounter T in a micro computer A for counting the number of the tightenedscrews when the user drives the electric screwdriver D in reversedirection to untighten the screw that has been tightened incorrectly asdiscussed above. To address this, it may be contemplated that thecircuit configuration of the electric screwdriver D is modified as suchas shown in FIG. 10. Specifically, the normal/reverse rotation selectionswitch RS is moved from the motor power source lines D1 and D2 to signalline D5 and replaced by an ON/OFF switch, wherein the power sourcecircuit in the controller P may be provided with normal/reverse changingfunction. In this modification, to cause the electric screwdriver D torotate in reverse direction, the normal/reverse rotation selectionswitch RS may be turned to ON position (closed state), for example, soas to send a signal (ON signal) of the reference voltage Vs from thecommon line D6 to the power supply circuit S in the controller P throughthe signal line D5 as a reverse rotation control signal, and to causethe power supply circuit S to invert the polarity of the output voltage(motor driving voltage), while at the same time sending the same ONsignal to the counter T to reverse the count back.

However, such a configuration would lead to some disadvantages,including a problem of cost increase in association with the increasednumber of signal lines in the electric screwdriver and another problemof no compatibility that an existing electric screwdriver is no moreapplicable to the controller having been modified as such as describedabove.

SUMMARY OF THE INVENTION

The present invention is directed to solve the above-problems.Specifically, the present invention provides an electric screwdrivercontroller having a function for determining that a correct tighteningoperation has been carried out when said tightening operation has takena time longer than a reference time. The controller comprises:

-   -   a torque monitoring means for monitoring whether or not a load        torque in carrying out a tightening operation by using an        electric screwdriver has reached a tightening completion torque        indicating the completion of the tightening operation;    -   a timer means for measuring a time interval from a starting time        of the tightening operation to a time when the tightening torque        has reached the tightening completion torque, as a tightening        operation time; and    -   a means for determining a reference time for a subsequent        tightening operation based on a plurality of tightening        operation time obtained from said timer means in the tightening        operations carried out for a predetermined number of screws.

This electric screwdriver controller may specifically comprise:

-   -   an expected time setting means for setting an expected time from        the starting time to the completion time of the tightening        operation;    -   a comparing means for comparing the tightening operation time        obtained from said timer means to said expected time for each of        the tightening operations; and    -   an expected time renewing means operable, when a comparison        result indicating that the tightening operation time is not        longer than the expected time is given by said comparing means,        to renew said expected time by replacing said expected time with        said tightening operation time, wherein the expected time        obtained from the expected time renewing means at the time of        completion of the tightening operations for a predetermined        number of screws is taken as the reference time.

More specifically, an electric screwdriver controller according to thepresent invention may comprise:

-   -   a time block setting means for setting a predetermined time        period as a time block;    -   a tightening completion torque setting means for setting a        tightening completion torque; and    -   a clock initialization means for initializing said timer means.    -   Said timer means comprises:    -   a time block processing means for starting a time block        processing in response to a starting of driving operation of        said electric screwdriver; and    -   a time block addition means for performing a time block        processing again by adding by one time block at each time when        the period for one time block has elapsed as long as said torque        monitoring means determines that said tightening torque has not        yet been reached said tightening completion torque.

Further, the present invention provides an electric screwdriver devicecomprising an electric screwdriver and a controller connected to saidelectric screwdriver.

-   -   Said controller comprises:    -   a power source and a power circuit for said power source, said        power circuit having a function for performing a switching        operation of the polarity of a power source terminal of said        power source between a state for a normal rotation and a state        for a reverse rotation in accordance with a normal rotation        signal or a reverse rotation signal generated by said electric        screwdriver and for shutting down an output from said power        source in accordance with a stop signal of said electric        screwdriver or a tightening completion signal generated by said        electric screwdriver; and    -   a tightening completion signal and reverse rotation signal        processing means for processing said tightening completion        signal and said reverse rotation signal.    -   Said power source may be a direct current power source.    -   Said electric screwdriver comprises:    -   a screwdriver motor having a power source connecting +/−line to        be connected to the power source terminal of said power circuit;    -   a common line for supplying a reference voltage;    -   a pair of power source lines for connecting said power circuit        of said controller to said screwdriver motor;    -   a normal rotation/reverse-rotation shifting switch for shifting        between the normal rotation and the reverse rotation of the        screwdriver motor;    -   a first signal line for interconnecting a first output terminal        of said normal rotation/reverse-rotation shifting switch and        said controller;    -   a second signal line for interconnecting a second output        terminal of said normal rotation/reverse-rotation shifting        switch and said controller;    -   a starting switch connected between said common line and a first        input terminal of said normal rotation/reverse-rotation shifting        switch; and    -   a braking switch connected in parallel with said starting switch        between said common line and a second input terminal of said        normal rotation/reverse-rotation shifting switch.    -   Said normal rotation/reverse-rotation shifting switch is capable        of switching between a normal rotation connecting position where        it is operable to connect said starting switch to said first        signal line and to connect said braking switch to said second        signal line, and a reverse rotation connecting position where it        is operable to connect said starting switch to said second        signal line and to connect said braking switch to said first        signal line.    -   Said starting switch is adapted:    -   to supply a start signal and a stop signal from said first        signal line to said controller, respectively, by turning ON and        OFF during said normal rotation/reverse-rotation switch being        positioned in said normal rotation connecting position; and    -   to supply the start signal and the stop signal from said second        signal line to said controller, respectively, by turning ON and        OFF during said normal rotation/reverse-rotation switch being        positioned in said reverse-rotation connecting position.    -   Said braking switch is adapted:    -   in case of it being a normal-open type switch, to be closed when        the tightening completion torque for the tightening operation        having been reached and to supply a tightening completion signal        to said controller; and    -   in case of it being a normal-close type switch, to be opened        when the tightening completion torque for the tightening        operation having been reached and to supply a tightening        completion signal to said controller.

This electric screwdriver device may be configured specifically suchthat said tightening completion signal and reverse rotation signalprocessing means comprises a tightening operation counter adapted tostart counting upon receipt of the tightening completion signal and toreverse the counting upon receipt of the reverse rotation signal.

Further, the present invention provides an electric screwdriver devicecomprising an electric screwdriver and a controller connected to saidelectric screwdriver. Said controller has a power circuit comprising apower source. Said electric screwdriver has a screwdriver motor having apower source connecting +/− line to be connected to a power sourceterminal of said power circuit. Said power source may be adirect-current power source. said screwdriver motor may be adirect-current motor of brush type.

-   -   said power circuit has a power circuit control section which        comprises:    -   a means for shutting down said power circuit to said motor in        response to a receipt of the tightening completion signal or the        stop signal of said electric screwdriver generated by said        electric screwdriver;    -   a means for forming a braking short circuit including said motor        in response to a receipt of the tightening completion signal or        the stop signal of said electric screwdriver generated by said        electric screwdriver; and    -   a control means for providing an ON/OFF control of said braking        short circuit.    -   Said control means is adapted to repeat the ON and the OFF        states alternately in a manner that periods of the OFF states        are kept constant and periods of the ON states are made longer        gradually with the number of revolutions of said motor is        reduced.

This electric screwdriver device may be configured specifically suchthat said screwdriver motor may be a direct-current motor of brush type,and said power circuit has a power circuit control section.

-   -   Said power circuit control section comprising:    -   a means for shutting down said power circuit to said motor in        response to a receipt of the tightening completion signal or the        stop signal;    -   a means for forming a braking short circuit including said motor        in response to a receipt of the tightening completion signal or        the stop signal; and    -   a control means for providing an ON/OFF control of said short        braking circuit. Said motor may be a direct-current motor.    -   Said control means is adapted to repeat the ON and the OFF        states alternately in a manner that periods of the OFF states        are kept constant and periods of the ON states are made longer        gradually with the number of revolutions of said motor is        reduced.

Since the present invention has made it possible to generate the signalfor reversing a counting by the counter for counting the number oftightened screws in the controller without increasing the number ofsignal lines as compared to the conventional electric screwdriver, saidsignal being required when a screw has not been correctly screwed-downand that screw has been rotated reversely and removed, therefore anycomplication and cost increase, which otherwise would be caused byincreased number of lines, could be avoided.

Further, the present invention allows for the expected time oftightening (reference time) to be taken as a reference for determiningwhether or not the tightening operation has been successfully completedto be determined automatically by simply conducting a tightening testfor a certain number of screws rather than by using a stopwatch aspracticed conventionally, and thus determined expected time can be setin the controller.

Furthermore, since in the present invention, in applying the ON/OFFcontrol to the braking short circuit to be formed at the time of thecompletion of the tightening operation, the OFF time is kept for acertain period but the ON time is varied in dependence on the number ofrevolutions of the motor or the magnitude of the current flowing throughthe braking short circuit, therefore the braking short circuit can beoperated efficiently so as to provide an efficient braking in a shorttime. In the conventional device, since the ON/OFF control has beencarried out by PWM (pulse width modulation), where the motor is in ahigh-speed revolution and an excessively high current is flowing,therefore the ON is short and consequently the OFF is made longer,resultantly inhibiting the efficient braking, which circumstance is tobe improved by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electric screwdriver device accordingto the present invention;

FIG. 2 is an electric diagram of the same device;

FIG. 3 is a flow chart illustrating a procedure for determining anexpected time (reference time) for the completion of the tighteningoperation to be carried out in the same device;

FIG. 4 is a circuit diagram in a normal rotation mode of a screwdrivermotor in the same device;

FIG. 5 is a circuit diagram in a reverse rotation mode of the samescrewdriver motor;

FIG. 6 is a circuit diagram in a tightening completion mode in the samescrewdriver motor;

FIG. 7 shows an example of an ON/OFF signal used in an ON/OFF control toa braking short circuit to be formed in the tightening completion modeas shown in FIG. 6;

FIG. 8 a plots a current value flowing through the braking short circuitfor the case of no ON/OFF control provided to the short braking circuit;

FIG. 8 b plots the current value flowing through the braking shortcircuit for the case of ON/OFF control provided to the braking shortcircuit;

FIG. 9 is a circuit diagram of an electric screwdriver device accordingto the prior art;

FIG. 10 shows a circuit diagram contemplated in a case where theelectric screwdriver device of FIG. 9 is to be provided with a functionfor performing a reverse counting by a counter for counting the numberof tightened screws when the screwdriver motor being rotated reversely;and

FIG. 11 is a circuit diagram that has been initially developed by theinventors of the present invention in order to improve the electriccircuit of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an electric screwdriver device according to thepresent invention will now be described with reference to the attacheddrawings.

As shown in FIG. 1, an electric screwdriver device 10 according to thepresent invention comprises an electric screwdriver 12 and a screwdrivercontroller 14 connected to the electric screwdriver via a cable 13 andhaving a function for preventing any screws from being left nottightened.

The electric screwdriver 12 has an electric circuit (electricscrewdriver circuit) as shown in FIG. 2 in association with the functionfor preventing any screws from being left not tightened. The illustratedelectric screwdriver circuit is intended to solve the problems inconjunction with the prior art as set forth above with reference toFIGS. 9 and 10. The circuit comprises power source lines D1 and D2 forsupplying a drive voltage to a screwdriver motor M, a first and a secondsignal lines D3 and D4 for sending an appropriate signal from theelectric screwdriver 12 to the controller 14, and a common line D5 forgenerating an appropriate signal onto the first and the second signallines D3 and D4. The common line D5 is connected to a reference voltageVs.

Further, this electric screwdriver circuit has a normalrotation/reverse-rotation shifting switch 28. The normalrotation/reverse-rotation shifting switch 28 has fixed contacts 28 a, 28b, 28 c and 28 d and movable contacts 28 e and 28 f. The fixed contact28 a is connected to the fixed contact 28 d and the fixed contact 28 bis connected to the fixed contact 28 c. One of the movable contacts 28 fis connectable to the common line D5 via a starting switch 20, and theother of the movable contacts 28 e is connectable to the common line D5via a braking switch 26 and the starting switch 20. The normalrotation/reverse-rotation shifting switch 28 is adapted to be manuallyshifted by an operator between a normal rotation connecting position(FIG. 2) where the movable contacts 28 e and 28 f are in contact withthe fixed contacts 28 c and 28 b, respectively and a reverse rotationconnecting position (not shown) where the movable contacts 28 e, 28 fare in contact with the fixed contacts 28 a and 28 b, respectively.

The starting switch 20 is an ON/OFF switch intended to send a start/stopsignal in response to the manual operation by the operator to thecontroller 14. If the starting switch 20 is closed (ON) under thecondition where the normal rotation/reverse-rotation shifting switch 28is in the normal rotation connecting position as illustrated, the signal(ON signal) at the reference voltage Vs from the common line D5 is sentfrom the starting switch 20 to the first signal line D3 via the movablecontact 28 f and the fixed contacts 28 d and 28 a of the normalrotation/reverse-rotation shifting switch 28, and through the firstsignal line D3, the signal is passed to a power circuit 32 of thecontroller 14 as the start signal. A microcomputer 40 in the controller14 is shifted to the normal rotation mode in response to a receipt ofthe start signal through the first signal line D3. If the startingswitch 20 is shifted to an open state (OFF), the signal at the referencevoltage Vs from the common line D5 is shut off, and this is given as thestop signal to the power circuit 32 of the controller 14.

On the other hand, if the starting switch is turned into the closedstate (ON) under the condition where the normalrotation/reverse-rotation shifting switch 28 is in the reverse rotationconnecting position, the signal (ON signal) at the reference voltage Vsfrom the common line D5 is sent from the starting switch 20 to thesecond signal line D4 via the movable contact 28 f and the fixed contact28 b of the normal rotation/reverse-rotation shifting switch 28, andthrough the second signal line D4, the signal is passed to the powercircuit 32 of the controller 14 as the start signal (i.e., the reverserotation signal). Further, the microcomputer 40 of the controller 14 isshifted to the reverse rotation mode in response to the receipt of thesame ON signal through the second signal line D4, and a counter forcounting the number of tightened screws in the microcomputer 40 now inthis mode carries out the reverse counting by decreasing the count valueby one. Furthermore, in the reverse rotation mode, as is the case withthe normal rotation mode, if the starting switch 20 is shifted to theopen state (OFF), the ON signal at the reference voltage Vs from thecommon line D5 is shut off thereby providing the stop signal to thepower circuit 32 of the controller 14.

The braking switch 26 is a normal-open type switch and adapted to beshifted to the closed state by a cam 29 incorporated in the screwdriver12, which is activated when the load torque in the tightening operationhas reached a predetermined value. When the braking switch 26 is closed,the tightening completion signal is sent from the screwdriver 12 to thecontroller 14. More specifically, when the braking switch 26 is closedwhile the normal rotation/reverse-rotation shifting switch 28 is in thenormal rotation connecting position as illustrated, the signal at thereference voltage Vs from the common line D5 is sent from the brakingswitch 26 to the second signal line D4 via the movable contact 28 e andthe fixed contacts 28 c and 28 b of the normal rotation/reverse-rotationshifting switch 28 as the tightening completion signal and through thesecond signal line D4, the signal is sent to the screwdriver controller14. In the screwdriver controller 14, the power circuit 32 is activatedin response to the tightening completion signal from the second signalline D4 to shut down the output of the motor driving voltage and to makea short circuit of the power source lines D1 and D2 to apply the brakingaction to the screwdriver motor M. In conjunction with this, the counter34 for counting the number of tightened screws in the microcomputer 40is caused to increase the count value by one.

It is to be noted that although FIG. 2 shows the configuration in whichthe start signal, the stop signal and the tightening completion signalare to be supplied directly from the respective signal lines to thepower circuit 32 within the screwdriver controller 14, anotherconfiguration may be contemplated, in which the microcomputer 40 oncereceives all of the signals from the electric screwdriver 12 so as tocontrol the power circuit 32.

In the tightening operation, when the normal rotation/reverse-rotationshifting switch 28 is placed in the normal rotation connecting positionas illustrated and the starting switch 20 is turned ON, then startsignal (reference voltage Vs) is sent to the power circuit 32 within thecontroller 14 via the first signal line D3, and the motor drivingvoltage having the polarity for the normal rotation is sent from thepower circuit 32 to the motor M via the power source lines D1 and D2.

When the motor M is driven in the direction for normal rotation to carryout the tightening operation and subsequently the load torque hasreached a predetermined value (i.e., when the tightening operation hasbeen completed), the braking switch 26 is closed by the cam 29 (which isactivated by the rotationally driven shaft of the electric screwdriver),and the tightening completion signal is sent to the power circuit 32 andthe microcomputer 40 (and thus to the counter 34 for counting the numberof tightened screws) within the screwdriver control device 14. In thepower circuit 32, any required actions, such as stopping or shutting offof the output of the motor driving voltage, is carried out, while in thecounter 34, the counting is carried out under the assumption that one ofthe screws has been tightened. The counting result (i.e., the countvalue or the accumulated number of screws that have been tightened) isindicated in a display 36.

If a wrong screw has been tightened, the operator may shift the normalrotation/reverse-rotation shifting switch 28 to the reverse-rotationconnecting position so as to close the starting switch 20 and thus tocause the starting signal (i.e., the reverse rotation signal) to be sentvia the second signal line D4 and thereby cause the power circuit 32 ofthe controller 14 to invert the output polarity, which in turn drivesthe motor M in the direction for reverse rotation to turn the screw,that has been once tightened, in the loosening direction for removingthe screw. At the same time, the reverse rotation signal is sent to thecounter 34, where the count is reversed to modify the counting of thenumber of tightened screws.

As set forth above, in the circuit wiring of the electric screwdrivershown in FIG. 2, the present invention allows the counter for countingthe number of tightened screws in the controller to have a function forreversing the count in association with the reverse rotation of theelectric screwdriver without increasing a number of signal lines fromthe electric screwdriver to the controllers.

It is to be noted that the circuit wiring of the electric screwdriverillustrated in FIG. 2 has been developed in order to solve problems(which will be discussed below) found in the circuit wiring as shown inFIG. 11, which had been initially developed by the inventor of thepresent invention to solve the problems as pointed out with reference toFIGS. 9 and 10.

Specifically, in the circuit wiring of FIG. 11, the electric screwdriverhas power source lines D1 and D2 for the screwdriver motor M, the firstand the second signal lines D3 and D4, and the common line D5, like theone as shown in FIG. 2. The common line D5 is fixed at the referencevoltage Vs (e.g., 0 volt). Further, this electric wiring has the normalrotation/reverse-rotation shifting switch 28. The normalrotation/reverse-rotation shifting switch 28 includes the fixed contacts28 a, 28 b, 28 c and 28 d, and the movable contacts 28 e and 28 f. Thefixed contact 28 b is connected to the fixed contact 28 c. The movablecontact 28 f is connected to the common line D5 via the starting switch20, while the movable contact 28 e is connected to the common line D5via the braking switch 26. The normal rotation/reverse-rotation shiftingswitch 28 is adapted to be shift between the normal rotation connectingposition (FIG. 11) where the movable contacts 28 e and 28 f areconnected to the fixed contacts 28 a and 28 c respectively and thereverse rotation connecting position (not shown) where the movablecontacts 28 e and 28 f are connected to the fixed contacts 28 b and 28 drespectively. Respective actions in the controller 14 including theactivation, the reverse rotation, the normal counting and reversecounting of the number of tightened screws and so on based on theoperations of the starting switch 20, the braking switch 26 and thenormal rotation/reverse-rotation shifting switch 28 are carried out in asimilar manner to that in the circuit shown in FIG. 2.

The problems in this circuit wiring are those as described below. Thecam 29 has a raised portion which turns the braking switch 26 on whenany excessive loading occurs. After setting the normal rotation orreverse-rotation by the normal rotation/reverse-rotation shiftingswitch, the starting switch is turned ON to carry out the tighteningoperation. In the tightening operation, when the braking switch isturned on, occasionally the braking switch could be held in the ONposition by the raised portion of the cam. If the normal rotation orreverse rotation is set by the rotation/reverse-rotation switch underthe condition where the braking switch is held in the ON position, theCPU in the microcomputer of the controller 14 determines that in asubsequent program cycle, the normal rotation or reverse rotation hasbeen set by the rotation/reverse-rotation switch and the starting switchhas been turned on, thereby activating the screwdriver in spite of thefact that the starting switch is actually not turned on. In contrast tothis, in the circuit wiring of FIG. 2, even if the braking switch is insuch a condition as described above, the starting switch arranged inseries with the braking switch is held in the OFF position, and thus noproblem will occur.

Another feature of the electric screwdriver device according to thepresent invention will now be described. In the controller 14 of theelectric screwdriver device according to the present invention, areference value is determined in order to enable the determinationwhether or not the screw has been tightened correctly without beinginclined.

Specifically, in the electric screwdriver device according to thepresent invention, a tightening operation time is used as a reference tomake the determination whether or not the tightening operation has beencarried out correctly without causing the screw to be inclined or out ofupright. This takes advantage of the fact that if the screw has beentightened out of upright, the tightening torque is increased morerapidly than that in the normal tightening operation to generate thetightening completion signal earlier. In the present invention, in orderto determine the reference value for the tightening operation time, thetightening operation test is carried out for the predetermined number ofscrews prior to the actual tightening operation without requesting theoperator to conduct a cumbersome process, wherein the time consumed foreach correct tightening operation is measured and the minimal value istaken as the reference value for the tightening operation time. Themicrocomputer 40 in the controller 14 determines the reference value ofthe tightening operation time based on the tightening completion signaland the like from the electric screwdriver in this tightening operationtest in a manner as will be described below.

FIG. 3 is a flow chart illustrating a procedure by the microcomputer 40to determine the reference value for the tightening operation timeaccording to the present invention.

In the illustrated embodiment, parameters are previously set such as atime block of a certain duration (50 msec in the illustratedembodiment), an expected time “TIM1” to be consumed until the tighteningoperation having been completed, a number of screws “ms” on which thetightening test is to be conducted and a number of screws on which thetightening test has been already finished (0 in the illustratedembodiment). When the tightening test is started, firstly a value of atightening measuring time “TIM2” is initialized to zero (step S1). Whenthe starting switch 20 is turned on in the tightening operation of eachscrew, the starting signal sent over the first signal line D3 isreceived, and it is determined that the motor M of the electricscrewdriver has started the revolution (step S2). From this time in theflow of procedure, a timer processing or time measuring operation foreach arbitrarily preset time block (50 msec in the illustratedembodiment) is started (step S3), and this timer processing is carriedout repeatedly for every time block until the tightening completionsignal is received from the second signal line D4 or until the loadtorque has reached the predetermined value for the tightening completion(step S4), wherein at each timer processing, the tightening operationtime TIM2 is incremented by one time block (50 msec in the illustratedembodiment) (step S5). When it is determined by receiving the tighteningcompletion signal that the tightening completion torque has been reached(step S4), the process further determines whether or not the tighteningoperation time TIM2 is larger than the previously set expected time TIM1(step S6), wherein if the tightening operation time TIM2 is smaller thanthe expected time TIM1, the expected time TIM1 is replaced by thetightening operation time TIM2 which is treated as a new expected timeTIM1, or the TIM1 is renewed (step S7). Further, the number of testedscrews “m” is incremented by one (step S8). This process is carried outuntil the number of tested screw has reached the predetermined number ofscrews “ms” (step S9, S10), and the finally obtained TIM1 is set andstored as the reference value Re (step S11). This reference value Re istaken as the reference time to be used in determining whether or not thetightening operation has been successfully performed in the actualtightening operation to be conducted subsequently to the tighteningtest. It is to be noted that since if the expected time TIM1 is setshorter, then the reference time is set to be equal to the expected timeTIM1, the expected time TIM1 may be automatically set to a maximum valueof time that can be set by the expected time setting means or a timelonger than a longest tightening operation time which may be taken inthe tightening operation by the electric screwdriver. The descriptionwill now be directed to a third feature of the present invention. FIG. 4is a conceptual diagram of an H-shaped bridge circuit for driving themotor M of the electric screwdriver 12 provided in the power circuit 32.

Specifically, M in FIG. 4 represents a DC motor of brush type serving asthe screwdriver motor, 42 designates a positive power source voltageterminal for the direct-current power source, 44 is a first switchelement, 46 is a second switch element, 48 is a third switch element, 50is a fourth switch element and 54 is a power circuit controller section.The first switch element 44 is connected between the positive powersource voltage terminal 42 and one of the terminals of the DC motor M,and the third switch element 48 is connected between the positive powersource voltage terminal 42 and the other of the terminals of the DCmotor M. Further, the fourth switch element 50 is connected between theground terminal (negative power source voltage terminal) and the one ofthe terminals of the DC motor M, and the second switch element 46 isconnected between the ground terminal and the other of the terminals ofthe DC motor M. Each switch elements 44, 46, 48, 50 is made of, forexample, transistor and applied with the ON/OFF control by the powercircuit control section 54.

The power circuit control section 54, upon receipt of the start signalfor the normal rotation mode from the first signal line D3, controls thethird switch element 48 and the fourth switch element 50 to be OFF andthe first switch element 44 and the second switch element 46 to be ON soas to drive the motor M in the direction for normal rotation, as shownin FIG. 4.

In contrast, when the power circuit control section 54 receives thestart signal for the reverse rotation mode (the reverse rotation signal)from the second signal line D4, it controls the first switch element 44and the second switch element 46 to be OFF and the third switch element48 and the fourth switch element 50 to be ON so as to drive the motor Min the direction for reverse rotation, as shown in FIG. 5.

Further, the power circuit control section 54, upon receipt of thetightening completion signal from the second signal line D4 during itsnormal rotation, controls the second switch element 46 and the fourthswitch element 50 to be OFF and the first switch element 44 and thethird switch element 48 to be ON, as shown in FIG. 6. In this case, aclosed circuit is formed which includes the first switch element 44, thethird switch element 48 and the motor M, and the thus formed closedcircuit works as the braking short circuit to the motor M to applybraking to the motor M.

If the motor M is in a high speed revolution at the time of formation ofthis braking short circuit, an electromotive force of high voltage isgenerated by the motor M and a high intensity of current flows throughthe same circuit. In the light of the fact that a wear of the brush issignificant if the excessively high intensity of current flows throughthe DC motor of brush type, the present invention, in order to avoid theproblem, provides ON/OFF control or switching control to the firstswitch element 44 and/or the third switch element 48 (the third switchelement 48 in the embodiment of FIG. 6).

The present invention has employed a frequency modulated control methodin order to provide this ON/OFF control, as shown in FIG. 7. In thismethod, during an initial stage where the motor M is in the high speedof revolution, the first switch element 44 and/or the third switchelement 48 are/is controlled to make an ON/OFF action at high frequency(with a constant length of the OFF time period), which is graduallylowered to the ON/OFF action of frequency consistent with the revolutionspeed. Specifically, since when the motor M is in the high speed ofrevolution, the voltage generated by the motor M serving as thegenerator is high, the first switch element 44 and/or the third switchelement 48 are/is controlled to make the ON/OFF action at highfrequency. Since the OFF time period is kept constant, the time periodfor which the first switch element 44 and/or the third switch element 48are/is kept ON is relatively made shorter, and consequently the increasein the current flowing through the circuit can be reduced. At the stageof the revolution speed of the motor M being lowered, the first switchelement 44 and/or the third switch element 48 are/is turned ON/OFF atlower frequency. Although in this illustrated embodiment, the frequencyis defined in four different levels as shown in FIG. 7, a steplessmodulation may be also applicable. FIG. 8 shows a current flowingthrough the braking short circuit with no such ON/OFF control applied(FIG. 8 a) and a current flowing through the braking short circuit withsuch ON/OFF control applied (FIG. 8 b) according to the presentinvention. As seen from this, when the ON/OFF control is appliedaccording to the present invention, the maximum current value flowingthrough the short circuit can be suppressed into the order of 4A byapplying the short circuit current at a predetermined time interval.

Although the present invention has been described with respect to someexemplary embodiments, the present invention is not limited thereto. Forexample, although the controller 14 comprises the counter 34 forcounting the number of tightened screws in the illustrated embodiment,the counter may be fitted externally, and in that case the controller isrequired only to send the tightening completion signal and/or thereverse rotation signal received from the electric screwdriver to theexternally fitted counter. Further, although in the illustratedembodiment, the starting switch 20 is applied with the referencevoltage, it may be applied with the ground potential.

1. A controller for use with a screwdriver having a drive shaft and anelectric motor activated to tighten a screw, the controller beingconfigured to determine whether or not a correct tightening operationhas been carried out, by comparing a time taken to complete thetightening operation and a reference time, the controller comprising: atorque monitor configured to monitor whether or not a load torqueexerted by the screwdriver for the tightening operation has reached at atightening completion torque; a timer configured to measure a tighteningoperation time which starts at a beginning of the tightening operationand ends when the tightening torque has reached at the tighteningcompletion torque; a reference time setter configured to determine thereference time for use in subsequent tightening operations, based on aplurality of tightening operation times measured by the timer; anexpected time setter configured to initially define an expected time tocomplete the tightening operation; a comparator configured to compare atightening operation time against the expected time, every time thetimer measures the tightening operation time taken to complete arespective tightening operation; and an expected time renewer configuredto define the measured tightening operation time as a new expected time,each time the comparator determines that the measured tighteningoperation time is not longer than the expected time, wherein thereference time is defined with the expected time calculated from thetightening operation times taken to complete a plurality of thetightening operations performed.
 2. A controller in accordance withclaim 1, wherein the expected time is initially set with one of amaximum value of time countable by the expected time setter and a timelonger than a longest tightening operation time ever taken for thetightening operation.
 3. A controller in accordance with claim 1,further comprising a time block setter configured to define a block oftime, wherein the timer starts to count a block of time upon a start ofthe tightening operation and continues counting the block of time untilthe tightening torque reaches at the tightening completion torque.